Fast frequency hopping is difficult to implement using conventional PLL frequency generators. Frequency generators that provide for fast frequency hopping purposes are required for example in receivers and transmitters for wireless communication such as WiMedia Ultra-Wide Band, where the hopping transient must be shorter than 9.5 ns.
Conventional PLL frequency generators synthesize signals of adjustable frequency by locking the phase of a frequency divided oscillator signal to a reference signal. Changes in the resulting frequency are realized by changing the frequency division ratio. After such changes a transition period typically occurs, before the PLL locks on to the new frequency. As a result, there are limitations on the achievable speed of frequency hopping.
A solution to this problem employs a plurality of phase-locked loops, and a multiplexer to provide an output signal from a selectable one of these loops. In this case different phase locked loops can be made to lock at different frequencies before their output signals are selected, thus avoiding a transition period after selection. Phase locked loops for all necessary frequencies may be used. When the required frequency hops are known in advance two phase locked loops may suffice, of which one is switched to the next frequency while the other still supplies the current frequency.
Such solutions with a plurality of phase locked loops are technically complex and require considerable circuit area and power consumption.